1. Field of the Invention
This invention relates to a method and apparatus for temperature sensing using fiber grating loop ringdown (FGLRD), in which a fiber-Bragg grating (FBG) or Long-Period grating (LPG) serves as the sensing element which determines temperature by measuring change of a time constant (i.e., ringdown time) when the element is exposed to temperature changes.
2. Related Art
Optical fiber sensors are superior to the conventional electrical-based sensors in many aspects: a) optical fiber sensors are inexpensive, compact, light weight, and immune to electromagnetic interference; and b) when employed in the measuring environment, they do not generate electrical hazards (1). In recent years, the development of fiber optic temperature sensors has been mainly based on the interference concept, such as the fiber Fabry-Perot interference (FFPI). There are several methods to design and fabricate an FFPI cavity in order to enhance detecting and reduce costs, but the basic principle remains the same: external changes of temperature induce variations of the fiber refractive index, fiber length, or both, thereby the resultant phase shift of the two coherent light beams changes. The temperature is measured through processing the interference patterns (4,5). Another type of fiber temperature sensor based on fiber-Bragg gratings (FBGs) has recently emerged (18). With these sensors, either a single grating or multiple gratings are written on a small section of the fiber. When the wavelength of the light source injected into the fiber satisfies the Bragg condition, the light of this wavelength is strongly reflected while the light of other wavelengths is transmitted. Both the FFPI temperature sensors and the FBG-based temperature sensors mentioned above are based on measuring the change of the light intensity to determine temperature. The disadvantages of the FFPI sensors include the costly and complicated fabrication of the interference cavity. Furthermore, the interference cavity, typically coated with chemicals, cannot survive in high temperature or chemically corrosive measuring environments. In order to increase the measurement accuracy these types of sensors take long integration time to process the interference patterns. The disadvantages of the current FBG sensors are low measurement accuracy and small measuring dynamic range. The measurement accuracy is limited by both the spectral bandwidth of the FBG and the spectral resolution of an optical spectral analyzer (OSA) or spectrometer. An OSA or spectrometer of high spectral resolution is often very expensive. The measurement accuracy of a typical FBG sensor using a bare single mode fiber is only ˜1° C. Alternatively, such sensors must use an expensive high resolution OSA and/or employ special fiber materials in order to improve the measurement accuracy of the device. Moreover, currently available products based on bare FBGs can only measure up to 200-300° C. Therefore, it is of great interest to explore new methods to develop fiber optic temperate sensors to be of low cost and high measurement accuracy, yet possess large measuring dynamic range.
Since its introduction (4), the cavity ringdown (CRD) technique has gained rapid development. The new ideas and the latest technologies have prompted the evolution of the cavity ringdown spectroscopy (CRDS) technique from the initial mirror-based CRD (4) to the reflection-based prism-CRD (5,6) and fiber end-coated CRD, as well as the very recently developed fiber-Bragg grating CRD (7,8,9). Although the cavity configurations are different, all of these CRD techniques can be classified as the high finesse cavity-based CRDs. Thus far, no documentation has been discovered that indicates these CRD techniques have been utilized for physical sensor development, partially due to practicality considerations resulting from the delicate and expensive coatings/polishing of the optical cavity. In the last couple of years, a new type of CRD technique, fiber loop ringdown (FLRD), has emerged. This technique adopts the CRD concept, but does not require the use of a high reflectivity mirror. Stewart et al. (11) first reported their work three years ago, in which a complicated fiber loop configuration was developed for a direct gas phase ringdown absorption measurement. Recently, Loock's (12,13) group advanced this technique using a simplified approach with a micro air-gap fabricated in the loop for liquids detection. Also, Lehmann et al. (14,15) explored FLRD for the detection of liquid samples based on evanescent field absorption. Research and development of this newly emerged FLRD technique itself is just at the starting point; measurements using the FLRD technique have been, thus far, limited to spectroscopic measurements or chemical sensing, e.g., detecting small volumes of liquids. However, owing to it ruggedness, low cost, and versatility, a variety of application potentials of the FLRD can be expected, including, but not limited to, development of FLRD systems and apparatus for physical sensor development (4, 5, 16, 23).
FBG has been incorporated into a section of optical fiber for FBGs CRD spectroscopic study (12). In that technique (FBGCRD), two FBGs are written in a section of optical fiber to form a cavity. A laser beam is emitted into the cavity through one FBG and leaks into a detector through the other FBG. At the Bragg wavelength region, due to the high reflectance of the FBGs, the light travels between the two FBGs. However, FBGs used in this manner function only to replace the pair of high reflectivity mirrors employed in the mirror-based CRD.
U.S. Pat. No. 6,563,970 provides a method and apparatus for a pressure sensor with a fiber-integrated fiber-Bragg grating system, also comprising a fiber-integrated fiber-Bragg grating temperature sensor. Additional methods for using fiber optic based temperature measurement have been described by Christian, et al. (U.S. App. No. 2001/0022804) and Tsao, Shyh-Lin, et al. (U.S. App. No. 2003/0072005).
3. Background of the Technology
Optical fiber temperature sensors offer unique advantages over the electrical-based temperature sensors in the following aspects: optical fiber-based temperature sensors do not generate electromagnetic hazards, have a large volume of data throughput, and facilitate easy site deployment and maintenance. There are a variety of approaches to develop optical fiber-based temperature sensors (1). Typical methods currently employed can be classified into two major categories: the chemical-based and the physical-based. The former includes using chemical coating, chemical fluorescence, and chemical doping; the latter includes utilizing the Fabry-Perot interference (FFPI) (2) and the fiber-Bragg gratings (FBGs) (3). Due to the non-chemically related measuring mechanism and the high detection sensitivity, the physical-based temperature sensors, such as FFPI and FBGs sensors, are very attractive in real applications. However, in extreme environment applications (e.g., NASA/space deployment), where the temperature measurements are performed under extremely harsh conditions, high pressure (˜8000 psi), high velocity fluids, very limited deployment space (e.g., a pipe of 8-10 inches inner diameter), and extreme temperatures (−170° C. to 500° C.), the FFPI and FBG temperature sensors become less practical. The FFPI-based temperature sensors cannot survive in the special measuring environments due to the chemical coating used in the FFPI cavity. The FBG-based temperature sensors measure the reflected spectral patterns to determine the temperature; the sensitivity and the accuracy totally depend upon the grating bandwidth and the resolution of a spectral analyzer. One of the significant limitations of the FBGs sensors is the low temperature accuracy which is limited by the spectral resolution of the OSA. Furthermore, the temperature measuring range of the FBG-based temperature sensors utilize an optical spectral analyzer (OSA), which is typically rather expensive, for the data processing, and the time response is relatively slow (>>400 ms).
Temperature sensors to be used, for example, in large rocket engine testing, must be non-intrusive, chemically clean, free of generating electromagnetic hazards, and capable of operating in extreme pressure and temperature environments. In many other applications, fiber optical temperature sensors are required to be of high measurement accuracy, fast response, low cost, as well as large dynamic measuring range.
The present invention satisfies these needs, as well as others.